The importance of the vitamin D in the biological systems of higher animals has been recognized since its discovery by Mellanby in 1920 (Mellanby, E. (1921) Spec. Rep. Ser. Med. Res. Council (GB) SRS 61:4). It was in the interval of 1920-1930 that vitamin D officially became classified as a "vitamin" that was essential for the normal development of the skeleton and maintenance of calcium and phosphorous homeostasis.
Studies involving the metabolism of vitamin D.sub.3 (cholecalciferol) were initiated with the discovery and chemical characterization of the plasma metabolite, 25-hydroxyvitamin D.sub.3 [25(OH)D.sub.3 ] (Blunt, J. W. et al. (1968) Biochemistry 6:3317-3322) and the hormonally active form, 1.alpha.,25(OH).sub.2 D.sub.3 (Myrtle, J. F. et al. (1970) J. Biol. Chem. 245:1190-1196; Norman, A. W. et al. (1971) Science 173:51-54; Lawson, D. E. M. et al (1971) Nature 230:228-230; Holick, M. F. (1971) Proc. Natl. Acad. Sci. USA 68:803-804). The formulation of the concept of a vitamin D endocrine system was dependent both upon appreciation of the key role of the kidney in producing 1.alpha., 25(OH).sub.2 D.sub.3 in a carefully regulated fashion (Fraser, D. R. and Kodicek, E (1970) Nature 288:764-766; Wong, R. G. et al. (1972) J. Clin. Invest. 51:1287-1291), and the discovery of a nuclear receptor for 1.alpha.,25(OH).sub.2 D.sub.3 (VD.sub.3 R) in the intestine (Haussler, M. R. et al. (1969) Exp. Cell Res. 58:234-242; Tsai, H. C. and Norman, A. W. (1972) J. Biol. Chem. 248:5967-5975). The operation of the vitamin D endocrine system depends on the following: first, on the presence of cytochrome P450 enzymes in the liver (Bergman, T. and Postlind, H. (1991) Biochem. J. 276:427-432; Ohyama, Y and Okuda, K. (1991) J. Biol. Chem. 266:8690-8695) and kidney (Henry, H. L. and Norman, A. W. (1974) J. Biol. Chem. 249:7529-7535; Gray, R. W. and Ghazarian, J. G. (1989) Biochem. J. 259:561-568), and in a variety of other tissues to effect the conversion of vitamin D.sub.3 into biologically active metabolites such as 1.alpha.,25(OH).sub.2 D.sub.3 and 24R,25(OH).sub.2 D.sub.3 ; second, on the existence of the plasma vitamin D binding protein (DBP) to effect the selective transport and delivery of these hydrophobic molecules to the various tissue components of the vitamin D endocrine system (Van Baelen, H. et al. (1988) Ann NY Acad. Sci. 538:60-68; Cooke, N. E. and Haddad, J. G. (1989) Endocr. Rev. 10:294-307; Bikle, D. D. et al. (1986) J. Clin. Endocrinol. Metab. 63:954-959); and third, upon the existence of stereoselective receptors in a wide variety of target tissues that interact with the agonist 1.alpha.,25(OH).sub.2 D.sub.3 to generate the requisite specific biological responses for this secosteroid hormone (Pike, J. W. (1991) Annu. Rev. Nutr. 11:189-216). To date, there is evidence that nuclear receptors for 1.alpha.,25(OH).sub.2 D.sub.3 (VD.sub.3 R) exist in more than 30 tissues and cancer cell lines (Reichel, H. and Norman, A. W. (1989) Annu. Rev. Med. 40:71-78).
Vitamin D.sub.3 and its hormonally active forms are well-known regulators of calcium and phosphorous homeostasis. These compounds are known to stimulate, at least one of, intestinal absorption of calcium and phosphate, mobilization of bone mineral, and retention of calcium in the kidneys. Furthermore, the discovery of the presence of specific vitamin D receptors in more than 30 tissues has led to the identification of vitamin D.sub.3 as a pluripotent regulator outside its classical role in calcium/bone homeostasis. A paracrine role for 1.alpha.,25(OH).sub.2 D.sub.3 has been suggested by the combined presence of enzymes capable of oxidizing vitamin D.sub.3 into its active forms, e.g., 25-OHD-1.alpha.-hydroxylase, and specific receptors in several tissues such as bone, keratinocytes, placenta, and immune cells. Moreover, vitamin D.sub.3 hormone and active metabolites have been found to be capable of regulating cell proliferation and differentiation of both normal and malignant cells (Reichel, H. et al. (1989) Ann. Rev. Med. 40: 71-78).
Given the pluripotent activities of vitamin D.sub.3 and its metabolites, much attention has focused on the development of synthetic analogs of these compounds. However, clinical applications of vitamin D.sub.3 and its structural analogs have been limited by the undesired side effects elicited by these compounds after administration to a subject, such as the deregulation of calcium and phosphorous homeostasis in vivo that results in hypercalcemia.